In digital oscilloscopes, consecutively sampled values of an applied waveform are digitized, stored in a memory, and then used to reconstruct the waveform as a displayable image (the “trace”) on a display device by reading and processing the stored values. The stored digital values are typically referred to as an acquisition record, the contents of which correspond to a definite time interval in the history of the applied waveform.
Many activities that are performed with an oscilloscope require that the displayed portion of the acquisition record be in some defined relationship to a detected event in the waveform, such as a rising or falling edge in the waveform, for example. The detected event is referred to as a trigger, or trigger event. When the event being detected is a condition of the waveform itself, the event is referred to as an internal trigger event. When the event being detected is a condition outside of the waveform that has some relationship to the waveform, such as another waveform, the event is referred to as an external trigger event. In response to a detected trigger event, some subset of the acquisition record is typically displayed to allow panning and zooming of the trace.
It is often desirable to capture data relative to multiple trigger events occurring in rapid succession, such as a set of successive radar pulses. This is often accomplished by using a mode of operations commonly referred to as “Segmented Memory Mode,” in which the data acquisition memory is divided into blocks of N samples. The data acquired for each trigger event is stored in a separate block of N-sample memory. In this mode, all memory segments are of the same length. In some applications, the amount of data that needs to be captured varies from trace to trace, but because all of the memory segments are of the same size, the digital oscilloscope must be set to capture the longest event of interest. This can waste large amounts of acquisition memory, requiring the user to make a tradeoff between the number of events captured and the length of the longest event that is captured. For example, one type of radar system outputs long pulses spaced far apart in time when the target is a long distance away. As the target moves closer, the radar pulses become shorter and closer together. Because the segment size must be set to capture the longest event, acquisition memory is wasted on the shorter events, and therefore fewer events can be captured.
Another method used in digital oscilloscopes to capture these types of trigger events is commonly referred to as the qualified storage method. With this method, the oscilloscope performs an algorithm that allows the user to supply a “qualification” signal that specifies to the oscilloscope when to store and not store data to acquisition memory. Aside from the time during which the oscilloscope samples the waveform, data storage is started and stopped instantaneously. This mode makes very efficient use of acquisition memory, but does not provide extra data before and after the trigger event to account for data lost due to signal processing. High-bandwidth digital oscilloscopes use various signal processing operations to increase the accuracy of the data, e.g., filtering, which reduces the amount of data available for the actual measurement. If the user wants extra data to be available when using the qualified storage method, the user has to manipulate the qualification signal in order to force the storage of the extra data. Although manipulating the qualification signal to store extra data at the end of the trigger event is easy to do, it is difficult or impossible to do at the beginning of the event.
A need exists for a measurement system that is efficient in terms of acquisition memory utilization, that allows a preselected amount of extra data to be easily stored in acquisition memory when performing a qualified store algorithm, and that does not require the user to manipulate the qualification signal in order to force the storage of the extra data.